A steel wire subjected to hot-dip galvanization for higher corrosion resistance, or a galvanized steel wire strand as a strand of such steel wires is used as a rope for use in a bridge. As a material for such a steel wire, for example, JIS G 3548 describes a steel wire having a wire diameter of 5 mm and a tensile strength TS of about 1500 to 1700 MPa. A carbon steel described in JIS G 3506 is mainly used as a material steel for the steel wire.
A steel wire as a material for the hot-dip galvanized steel wire is required to be reduced in manufacturing cost and to have higher strength. Higher strength advantageously reduces steel usage and improves the degree of freedom of bridge design.
The galvanized steel wire is typically manufactured in the following manner. First, a wire rod (steel wire rod) fabricated through hot rolling is placed in a ring shape on a cooling conveyer for pearlite transformation, and is then wound up into a coil to yield a wire rod coil. Subsequently, the wire rod coil is subjected to patenting treatment so as to have higher strength and a homogenous microstructure. The patenting treatment is a type of heat treatment, in which a wire rod is typically heated to about 950° C. using a continuous furnace and austenized, and is then dipped in a refrigerant such as a lead bath maintained at about 500° C. to produce a fine and homogeneous pearlite phase.
Subsequently, the wire rod is subjected to cold wire-drawing, so that a steel wire having a predetermined strength is produced by the effect of a work hardening function of pearlite steel. Subsequently, the steel wire is dipped in a galvanizing bath maintained at about 450° C. for galvanization, so that a galvanized steel wire is produced. The galvanized steel wire may be further subjected to finish drawing. A parallel wire strand (PWS) as a bundle of galvanized steel wires produced in such a way or a galvanized steel wire strand as a strand of such steel wires is used to produce a cable for a bridge.
In such a series of manufacturing steps, the patenting treatment causes an increase in manufacturing cost. Although the patenting treatment is effective in increasing strength of a wire rod and homogenizing quality thereof, the patenting treatment increases manufacturing cost, and has environmental problems such as CO2 emission and use of an environment-load substance. The hot-rolled wire rod could be advantageously drawn to be formed into a steel wire product without heat treatment such as the patenting treatment. Drawing the hot-rolled wire rod without heat treatment is generally called “rod drawing”.
A variation in strength in a longitudinal direction of the rod-drawn wire rod is an issue in achieving a high-strength rod-drawn wire rod. In a typical manufacturing process of a wire rod with air blast cooling, the wire rod is cooled while being placed in a ring shape on a cooling conveyer. FIG. 1 is a schematic illustration of a state of the ring-shaped wire rod on the cooling conveyer. Cooling the wire rod in such a state causes a portion of a dense part 10 in which portions of the wire rod lie relatively dense, and a portion of a sparse part 11 in which portions of the wire rod lie relatively sparsely.
As a result, cooling rate varies between the dense part 10 and the sparse part 11, and the precipitating pearlite phase has a periodic variation corresponding to a circumference of a ring; hence, the mechanical properties of the wire rod also have a periodic variation. When a wire rod has a variation in strength, product strength is designed with reference to the lower limit of the strength of the wire rod on the safety grounds. Hence, decreasing a variation in strength of the wire rod enables design of a product having higher strength. A rod-drawn wire rod does not get the benefit of homogenizing a microstructure by patenting treatment. Hence, the microstructure of such a wire rod must be homogenized through microstructure control after hot rolling to decrease the variation in strength.
There have been provided various techniques for improving wire-drawability. For example, PTL 1 provides a technique for improving wire-drawability through cooling in a molten salt bath after hot rolling. Such a technique is called direct patenting treatment.
PTL 2 discloses a technique for increasing strength of a wire rod by controlling a cooling condition after hot rolling so that the patenting treatment is omitted.
PTL 3 discloses a technique for improving wire-drawability of a spring-steel wire rod by decreasing a variation in pearlite phase depending on coil density.